Justice Anthony Kennedy, a Republican, hasn't toed the conservative line on gay rights. That may deter the Supreme Court from taking the Proposition 8 case.

Reporting from Washington — The Supreme Court has nine justices, but if the constitutional fight over same-sex marriage reaches them this year, the decision will probably come down to just one: a California Republican and Reagan-era conservative who has nonetheless written the court's two leading gay rights opinions.

JusticeAnthony M. Kennedy, 75, often holds the court's deciding vote on the major issues that divide its liberals and conservatives. More often than not, that vote has swung the court to the right. But on gay rights, Kennedy has been anything but a "culture wars" conservative.

One of his opinions lauded the intimacy between same-sex couples and demanded "respect for their private lives," provoking Justice Antonin Scalia to accuse him of having "signed on to the so-called homosexual agenda."

"He is a California establishment Republican with moderately libertarian instincts," Stanford University law professor Pamela Karlan said of Kennedy. "He travels in circles where he has met and likes lots of gay people."

Based on Kennedy's past opinions, Karlan is confident that if the Supreme Court takes up the issue of California's same-sex marriage ban, "it meansProp. 8is going down to defeat," she said. "There is no way he will take it to reinstate" the ban.

Not all court observers share her prediction, but the uncertainty about how Kennedy might vote may, by itself, be enough to deter the high court from hearing an appeal of the decision by the U.S. 9th Circuit Court of Appeals. Four justices must vote for the court to consider a case, but a majority is needed to issue a ruling.

When an appeal reaches the high court, the four most conservative justices will face a tough choice: Vote to have the court hear the case and run the risk that Kennedy would side with the more liberal justices to go beyond the 9th Circuit decision and establish a nationwide right to same-sex marriage. Or turn the case aside, leaving same-sex marriage intact in California but setting no national precedent.

The man at the center of the speculation grew up in a Catholic family in Sacramento, where his father was a lawyer and lobbyist in the Legislature. Family friends included then-Gov. Earl Warren. As a Harvard law student, the young Kennedy visited the Supreme Court to meet with Warren, who was then chief justice.

As a justice since 1988, Kennedy has reflected at times both styles of Republicanism: the conservatism and respect for states' rights of Reagan, who appointed him, as well as Warren's devotion to civil rights and fair treatment.

Two years ago he wrote the much-disputed 5-4 opinion in the Citizens United case that said corporations and unions had a free-speech right to spend freely on election campaigns. But also that year Kennedy wrote a 5-4 opinion that struck down as cruel and unusual punishment the laws in Florida and elsewhere under which juvenile offenders were sent to prison for life for crimes that did not involve a murder. Sounding a bit like Warren, Kennedy said it was unfair to close the prison doors forever on youths who had gone wrong.

Eight years ago he wrote the decision that declared unconstitutional laws in Texas and elsewhere that made gays subject to arrest for "deviate" sexual conduct. "The state cannot demean" same-sex couples by making their intimate, private conduct into a crime, Kennedy said.

In 1996, he wrote an opinion in a Colorado case called Romer vs. Evans that formed the basis for Tuesday's 9th Circuit decision striking down Proposition 8.

Colorado voters had approved an initiative that stripped gays and lesbians of civil rights protections under state and local ordinances. Kennedy said the law could not stand because it was "born of animosity" toward homosexuals and took away their hard-won legal rights.

In Tuesday's decision, Judge Stephen Reinhardt of Los Angeles did not say gays had a right to marry as a matter of equal treatment. Instead, he focused on same-sex marriage in California and repeated Kennedy's view that voters could not take away the rights gays had briefly won. "Prop. 8 singles out same-sex couples for unequal treatment by taking away from them alone the right to marry," Reinhardt wrote, citing Romer vs. Evans.

Kennedy sits in the middle of two ideological blocs likely to split evenly on the question of same-sex marriage. The four conservatives — Chief Justice John G. Roberts Jr.and Justices Scalia, Clarence Thomas and Samuel A. Alito Jr. — are likely to oppose the 9th Circuit's decision on the grounds that judges should not force such a change in state law. The four liberals — Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — are likely to support the 9th Circuit's decision as a matter of equal treatment.

"Both sides will be nervous," said Michael Dorf, a Cornell University law professor who has clerked for Reinhardt and Kennedy. The California-only approach taken by Reinhardt would allow the high court to pass up the case, but he and others predict the justices will hear it. "This legalizes same-sex marriage in the biggest state. That's a big deal in itself," Dorf said.

Chapman University law professor John Eastman said conservatives had not given up on Kennedy.

"I know some people say Justice Kennedy will ask: Should we stop the progress now? I think Justice Kennedy will ask: Do we want to put a stake in the heart of an institution, marriage, that has done so much for society?" he said.

Professor Erwin Chemerinsky, dean of UC Irvine Law School, believes Kennedy will play the crucial role and write a broader opinion that undercuts other state laws banning same-sex marriage. "This is a court that wants to have the last word on major legal issues," he said.